Optical guiding and loss mechanisms in electro-optically induced waveguides based on isotropic phase liquid crystals

Summary form only given. Electro-optical (EO) waveguides induced in EO materials by applying electric fields across defined regions, shown in Fig. 1(a), offer great potential for applications in good integrable and reliable photonic devices. From the variety of materials suitable for such so-called electro-optically induced waveguides (EOIW), very promising are certain nematic liquid crystals (LC) in their isotropic phase, which in particular near their clearing point exhibit large Kerr coefficients (about 10<;sup>-10<;/sup> m/V<;sup>2<;/sup>), transparency in visible-infrared range as well as short response times (<;1 μs) [1]. In this paper, a device based on the concept of EOIW in isotropic liquid crystals is designed and characterized. Experimental data on the fiber to EOIW coupling efficiency and corresponding losses are obtained and compared to results from FEM simulations. It is shown that at higher voltages the losses are dominated by the set-in of an electric field induced nematic-isotropic phase transition (see also [2]).2. Measured EOIW characteristics: The cross section of the EIOW based device is presented in Fig. 1(b). The EOIW extends to 1 cm in length. A device body (substrates with structured electrodes and cladding layers) was fabricated using wafer level silicon technology processes. In a last step, the LC was inserted into the device body. To assure that the electro-optic Kerr constants K are high, the EOIW chip was held at a temperature of 37 °C (this is just above the nematic to isotropic phase transition). By means of a polarization-maintaining fiber, light from a He-Ne laser (λ = 632.8 nm) was coupled into the EOIW, while the output signal was captured using a large area multimode fiber. In this configuration, the setup allows measuring Γ . T, where Γ is the coupling efficiency between the single mode fibre and the EOIW and T is the transmission coefficient. From the - esult shown in Fig. 1(c), it is evident that with increasing voltage the light transmission through the device (Γ . T) increases, reaches a maximum and then decreases. We attribute the decrease to the set-in of an induced phase transition from the isotropic back to the nematic phase. In order to understand this effect, simulations based on independently measured data are used to quantify the contributing loss and guiding mechanisms.3. Simulation of EOIW Eigenmodes: As indicated in Fig. 1(c), we attempt to explain the experimental data by cumulated contributions from loss Γ and transmission T. In order to obtain separate expressions for Γ and T contributions, the EOIW eigenmodes were calculated by a finite element method model, which permits the calculation of the refractive index tensor dependence on the electric field. In particular, from the field distributions and the imaginary part of the effective refractive indices, Γ and T could be determined. (For the calculation we used n = 1.555 and K = 5.4 . 10-11 m/V2, which have been measured for this LC for this purpose.). The imaginary part of the refractive index k gives the scattering loss due the electric field induced nematic phase. We consider k > 0 for field strengths E above the critical field strength Ek and k = 0 otherwise. For k = 3.5 . 10-3 and Ek = 1.4 . 10-7 V/m, the model Γ . T shows very good agreement with experimental data. 4. Conclusion: A model for guiding and loss mechanisms in EOIW devices was derived, which shows very good agreement with measured data. Thus, Γ and T could be separately investigated. The understanding of the underlying guide and loss mechanisms is indispensable for the development of further devices exploiting the benefits of EOIWs based on isotropic phase liquid crystals.